Recently, with the development of portable devices such as personal computers and portable telephones, batteries as a power source for such devices have been increasingly demanded. The batteries used for the above-mentioned applications are required to be used at ordinary temperatures as well as to have a large energy density and excellent cycle characteristics.
In order to respond to these requirements, for both positive electrode and negative electrode, active materials having a large capacity density have been newly developed. Above all, elemental substances, alloys, or oxides of silicon (Si) or tin (Sn) that can provide an extremely large capacity density are promising materials as negative active materials.
However, in lithium secondary batteries using these negative electrode active materials, sufficient charge/discharge cycle characteristics cannot be obtained. Specifically, silicon oxide “SiOx (x=0.3)” is used as the negative electrode active material. Lithium cobaltate (LiCoO2) used in a general lithium secondary battery may be used as the positive electrode active material. A mixture solution of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) containing lithium hexafluorophosphate (LiPF6) may be used as an electrolyte. These are used so as to produce a winding type lithium secondary battery. When this battery is charged and discharged repeatedly in the conditions of charge and discharge current of 1.0 C, charge end voltage of 4.2 V and discharge end voltage of 2.5 V, large capacity deterioration occurs at about 100 cycles.
In order to avoid the above-mentioned problem, it is preferable that the discharge electric potential at the negative electrode is made to be as low as possible. For example, Patent document 1 discloses a lithium secondary battery using silicon oxide (SiO) as a negative electrode active material in which capacity deterioration associated with the charge/discharge cycle can be suppressed by controlling the discharge end electric potential of the negative electrode to 0.6 V or less with respect to a lithium reference electrode.
However, as in Patent document 1, when the lithium reference electrode is provided in the battery in order to control the discharge electric potential of the negative electrode using SiO with respect to the lithium reference electrode, the battery configuration becomes complicated, thus making practicability thereof difficult. In other words, a battery needs to have a third electrode terminal, in addition to a positive electrode terminal and a negative electrode terminal. Furthermore, when a charge/discharge cycle is repeated from the beginning of the charge/discharge cycle with less deterioration of the positive and negative electrode active materials, electric potential ranges in which both positive and negative electrodes are used are changed according to the deterioration of active materials of the positive and negative electrodes. Therefore, even if the discharge end voltage is set so that the discharge end electric potential of the negative electrode becomes 0.6 V or less based on the positive electrode, it is difficult to determine whether or not the discharge end electric potential of the negative electrode is maintained to be 0.6 V or less. Thus, it is difficult to suppress the capacity deterioration associated with the charge/discharge cycle by measuring a battery voltage so as to detect a charge and discharged state of the negative electrode.    Patent document 1: Japanese Patent Unexarmined Publication No. H11-233155